The present invention relates to oligomeric organosilanes, a process for their production as well as their use.
It is known to use sulfur-containing organosilicon compounds such as 3-mercaptopropyltrimethoxysilane or bis-(3-[triethoxysilyl]-propyl) tetrasulfane as silane coupling agents or reinforcing additives in oxidically filled rubber mixtures, including inter alia for treads and other parts of automobile tires (DE 2 141 159, DE 2 212 239, U.S. Pat. Nos. 3,978,103, 4,048,206).
Rubber mixtures are known from EP 0 784 072 A1 that are based on at least one elastomer with silica as a filler and a reinforcing additive that is produced by mixing, or as an in situ reaction product, at least one functional polyorganosiloxane compound, and that contain a functional organosilane as a further constituent. As monomeric building blocks there are used in particular 3-mercaptopropyltrialkoxysilanes or bis (trialkoxysilylpropyl) tetrasulfanes that in each case carry 3 or 6 alkoxy substituents.
In the production of rubber mixtures with organosilanes and a filler, for example a precipitated silica, a chemical reaction takes place during a first mixing process, for example in an internal mixer. This chemical reaction involves a condensation between the organosilane and the filler, which is accompanied by a considerable release of alcohol. This eliminated alcohol causes to some extent considerable technical problems during the further processing of the rubber mixtures, such as mixture porosity in the extrusion or undesirable formation of bubbles in the rubber per se. Furthermore a reduction in the release of alcohol during the reaction is desirable for both health and environmental reasons.
It is known that these disadvantages can be largely avoided by the use of oligomeric organosilanepolysulfanes instead of the monomeric sulfur-containing organosilicon compounds that were hitherto used.
Oligomeric organosilanes that are built up from different tructural units A and/or B and/or C 
are known from EP 0964021.
A disadvantage of the known oligomeric organosilanes is the poor reinforcing behaviour in rubber mixtures.
An object of the present invention is to produce oligomeric organosilanes that have an improved reinforcing property in rubber mixtures and that on account of the oligomerization lead to a reduced release of ethanol during the mixing process and the subsequent processing steps.
The above and other objects of the present invention can be achieved by oligomeric organosilanes that are built up from the following 2 structural units A and B according to formula I, 
wherein R1, R2 denote a (C1-C4)alkoxy group, preferably a
methoxy or ethoxy group,
R3 denotes a straight-chain or branched (C1-C20)alkyl group, preferably a propyl, octyl or hexadecyl group,
n is equal to 1-8, preferably 3, and
o and p in each case denote a whole positive number from 1-40,
where p/o is equal to 0.2/1 to 6/1.
Where R3xe2x95x90C1-C5 then p/o may preferably be 2/1 to 5/1, where R3xe2x95x90C6-C8 then p/o may preferably be 0.5/1 to 3/1 and where R3xe2x95x90C9-C20 then p/o may preferably be 0.2/1 to 2/1.
The oligomeric organosilanes may be present as an individual compound with a defined molecular weight as well as an oligomeric mixture with a molecular weight distribution.
The oligomeric organosilanes can have molecular weights of 200 to 16000 g/mole. Preferably the oligomeric organosilanes according to the invention can have molecular weights of 400 to 5000 g/mole.
The invention also relates to a process for the production of the oligomeric organosilanes as described herein, which comprises forming a reaction mixture by mixing together the monomeric compounds of the structure types I and II 
in which R1, R2, R3 and n have the meanings given above and R4 is a (C1-C4)alkoxy group, preferably a methoxy or ethoxy group, in which R4 may be identical or different, and then co-oligomerizing the reaction mixture.
In this connection organosilicon compounds of arbitrary structure and with variously long sequences of the two structural units may be formed within the scope of the structure types I and II given above.
According to the invention, the co-oligomerization reaction may be carried out in a solvent and/or optionally with the aid of a catalyst, at a reaction temperature between 0xc2x0 C. and 150xc2x0 C.
As the organosilicon compound of the structural unit I there can be used mercaptopropyltriethoxysilane, mercaptopropyl-trimethoxysilane or mercaptopropyl-methyldiethoxysilane.
As the organosilicon compound of the structural unit II there can be used propyltriethoxysilane, propyltrimethoxysilane, propylmethyldiethoxysilane, dimethyldiethoxysilane, octyltriethoxysilane, octyltrimethoxysilane, hexadecyltriethoxysilane or hexadecyltrimethoxysilane.
The co-oligomerization can be carried out with the addition of water and the release of alcohol in bulk or in an inert organic solvent or mixtures thereof, such as for example in an aromatic solvent such as chlorobenzene, a halogenated hydrocarbon such as chloroform, methylene chloride, an ether such as diisopropyl ether, tert.butyl methyl ether, tetrahydrofuran or diethyl ether, acetonitrile or carboxylic acid esters, for example ethyl acetate, methyl acetate or isopropyl acetate, an alcohol, for example methanol, ethanol, n-propanol, i-propanol, n-butanol, sec. butanol or tert. butanol. Preferred solvents may be ethanol or ethyl acetate.
The reaction according to the present invention may be catalysed. The catalyst can be added in catalytic or stoichiometric amounts. In this connection all types of acidic, basic or neucleophilic catalysts that are known to the person skilled in the art from the SOLGEL chemistry of alkoxysilanes (see for example R. Corriu, D. Leclercq, Angew. Chem. 1996, 108, 1524-1540) are also suitable for the oligomerization within the context of the invention. The catalysts may be present in the same phase as the reaction solution (homogeneous catalysis) or may be present as solids (heterogeneous catalysis) and are separated after the end of the reaction.
An acidic, basic or nucleophilic catalyst can be used in the catalysis.
The basic catalysis can be carried out for example with an organic base such as triethylamine, tetramethylpiperidine, tributylamine or pyridine, or with an inorganic base such as NaOH, KOH, Ca(OH)2, Na2CO3, K2CO3, CaCO3, CaO, NaHCO3, KHCO3, or alcoholates such as NaOCH3 or NaOC2H5.
Nucleophilic catalysis can be performed with aluminum oxide or suitable fluorides, for example ammonium fluoride, sodium fluoride, potassium fluoride, or arbitrary tetraalkylammonium fluorides such as tetrabutylammonium fluoride.
Acid catalysis can be carried out with dilute aqueous mineral acids or solutions of Lewis acids in water. Tetrabutyl orthotitanate can for example be used as Lewis acid.
The catalysis is preferably carried out with dilute aqueous NaOH or a solution of ammonium fluoride in water, 1 mole % of catalyst being employed with reference to the amount of water used.
Suitable amounts of methanol can be added for the catalysis.
The reaction conditions, in particular the amount of water added, can be chosen so that the reaction products do not polycondense to form a solid.
After completion of the reaction the readily volatile constituents can be removed and the catalyst can be deactivated in a conventional manner or removed.
The present invention also relates to rubber mixtures which comprise rubber, fillers such as for example precipitated silica, optionally further rubber auxiliary substances, as well as at least one oligomeric organosilane according to the invention.
The oligomeric organosilane according to the invention can be used in amounts of from 0.1 to 15 wt. %, referred to the amount of the rubber used.
The addition of the oligomeric organosilanes according to the invention as well as the addition of the fillers can preferably take place at melt temperatures of 100xc2x0 to 200xc2x0 C. The addition can however also take place subsequently at lower temperatures (40xc2x0 to 100xc2x0 C.) for example together with further rubber auxiliary substances.
The oligomeric organosilanes can be added to the mixing process in pure form as well as applied to an inert organic or inorganic carrier. Preferred carrier materials are silicas, natural or synthetic silicates, aluminum oxide or carbon blacks.
As fillers the following substances can be used for the rubber mixtures according to the invention:
Carbon blacks: the carbon blacks used in this connection are produced by the flame black, furnace black or gas black process and have BET surface area of 20 to 200 m2/g, such as for example SAF, ISAF, HSAF, HAF, FEF or GPF carbon blacks. The carbon blacks can optionally also contain heteroatoms such as e.g. Si.
Highly dispersed silicas, produced for example by precipitating solutions of silicates or flame hydrolysis of silicon halides with specific surfaces of 5 to 1000 m2/g, preferably 20 to 400 m2/g (BET surface area) and with primary particle sizes of 10 to 400 nm. The silicas can optionally also be present as mixed oxides with other metal oxides such as Al, Mg, Ca, Ba, Zn and titanium oxides.
Synthetic silicates such as aluminum silicate, alkaline earth silicates such as magnesium silicate or calcium silicate, with BET surfaces of 20 m2/g to 400 m2/g and primary particle diameters of 10 to 400 nm.
Natural silicates such as kaolin and other naturally occurring silicas.
Glass fibers and glass fiber products (mats, strands) or microsize glass spheres.
There can preferably be used carbon blacks with BET surface areas of 20 m2/g to 400 m2/g or highly dispersed silicas produced by precipitation of solutions of silicates, with BET surface areas of 20 m2/g to 400 m2/g, in amounts of 5 to 150 parts by weight, in each case referred to 100 parts of rubber.
The aforementioned fillers can be used alone or as a mixture. In a particularly preferred embodiment of the process, for the production of the mixtures there can be used 10 to 150 parts by weight of light fillers, optionally together with 0 to 100 parts by weight of carbon black, as well as 0.3 to 10 parts by weight of a compound of the oligomeric organosilanes according to the invention, in each case referred to 100 parts by weight of rubber.
In addition to natural rubber, synthetic rubbers are also suitable for the production of the rubber mixtures according to the invention. Preferred synthetic rubbers are described for example in W. Hofmann, Kautschuktechnologie, Genter Verlag, Stuttgart 1980. Such rubbers include, inter alia,
polybutadiene (BR)
polyisoprene (IR)
styrene/butadiene copolymers with styrene contents of 1 to 60 wt. %, preferably 2 to 50 wt. % (SBR)
isobutylene/isoprene copolymers (IIR)
butadiene/acrylonitrile copolymers with acrylonitrile contents of 5 to 60 wt. %, preferably 10 to 50 wt. % (NBR)
partially hydrogenated or fully hydrogenated NBR rubber (HNBR)
ethylene/propylene/diene copolymers (EPDM)
as well as mixtures of these rubbers. Anionically polymerized L-SBR rubbers with a glass transition temperature above xe2x88x9250xc2x0 C. as well as their mixtures with diene rubbers are particularly suitable for the production of automobile tires.
The rubber vulcanisates according to the invention may contain further rubber auxiliary substances such as reaction accelerators, anti-ageing agents, heat stabilizers, light stabilizers, anti-ozonants, processing auxiliary substances, plasticizers, tackifiers, blowing agents, dyes, pigments, waxes, extending agents, organic acids, inhibitors, metal oxides as well as activators such as triethanolamine, polyethylene glycol, hexanetriol, that are known in the rubber industry.
The rubber auxiliary substances can be used in known amounts that are governed by, inter alia, the intended use. Conventional amounts are for example amounts of 0.1 to 50 wt. % referred to the rubber. Sulfur or sulfur-donating substances can be used as crosslinking agents. The rubber mixtures according to the invention can furthermore contain vulcanization accelerators. Examples of suitable vulcanization accelerators are mercaptobenzothiazoles, sulfenamides, guanidines, thiurams, dithiocarbamates, thioureas and thiocarbonates. The vulcanization accelerators and sulfur are used in amounts of 0.1 to 10 wt. %, preferably 0.1 to 5 wt. %, referred to the rubber.
The vulcanization of the rubber mixtures according to the invention can take place at temperatures of 100xc2x0 to 200xc2x0 C., preferably 130xc2x0 to 180xc2x0 C., optionally under a pressure of 10 to 200 bar. The mixing of the rubbers with the filler, optionally rubber auxiliary substances and the oligomeric silanes (I) according to the invention can be carried out in known mixing equipment such as rollers, internal mixers and mixer-extruders.
The rubber mixtures according to the invention are suitable for the production of moulded articles, for example for the production of pneumatic tires, tire treads, cable sheathing, hoses, drive belts, conveyor belts, roller coatings, tires, shoe soles, sealing rings and damping elements.
The oligomeric organosilanes according to the invention exhibit the advantages of a low rolling resistance (correlated with tan xcex4 60xc2x0 C.), improved abrasion resistance, improved scorch behaviour and higher reinforcing factor (M300/M100).